I think I found the solution for both problems because of an other thread: Design Pattern: Motion Sensor Timer
1.) Using "sendCommand(OFF) " instead of "sendCommand(new HSBType(new DecimalType(0),new PercentType(0),new PercentType(0))) " solves the flickering problem
2.) since using "Thread::sleep(xyz) " I had no unreacting bulbs so it seems that the problem was overlapping commands at the same time. I tested a little bit and found 300 ms are enough to execute several commands -> “Thread::sleep(300)” between each command is perfect.
Thanks